Method for forming cast-in-place caseless concrete piles

ABSTRACT

The method of forming cast-in-place caseless concrete piles where pile holes to be driven in the earth into which concrete is to be fed occupy a proximity area such that transmission of forces during driving of some pile holes may damage the shape of others, wherein a respective hollow tubular driving mandrel and driving foot member releasably coupled thereto are driven into the earth to form each pile hole in the proximity area, and the mandrel for driving each hole is left fully inserted in its pile hole to protect the shape of the pile hole until all pile holes in such proximity area are driven, and the mandrels are then withdrawn after flowing concrete in the pile holes to leave castin-place caseless piles.

United States Patent [1 1 LeCorgne Dec. 16, 1975 METHOD FOR FORMINGCAST-IN-PLACE CASELESS CONCRETE PILES [75] Inventor:

[73] Assignee: lnterpile USA, Inc., New Orleans,

[22] Filed: July 22, 1974 211 Appl, No.: 491,481

[52] US. Cl. 6l/53.64; 61/50; 61/565 [51] Int. Cl. EOZD 5/34; EO2D 3/08[58] Field of Search 61/50, 53.5, 53.52, 53.64,

{56] References Cited UNITED STATES PATENTS 3.638.433 2/1972 Sherard61/535 X 12/1974 Steding.....

William R. LeCorgne, New Orleans,

Primary Examiner-Jacob Shapiro Attorney, Agent, or Firm-Mason, Fenwick &Lawrence [57] ABSTRACT The method of forming cast-in-place caselessconcrete piles where pile holes to be driven in the earth into whichconcrete is to be fed occupy a proximity area such that transmission offorces during driving of some pile holes may damage the shape of others,wherein a respective hollow tubular driving mandrel and driving footmember releasably coupled thereto are driven into the earth to form eachpile hole in the proximity area, and the mandrel for driving: each holeis left fully inserted in its pile hole to protect the shape of the pilehole until all pile holes in such proximity area are driven, and themandrels are then withdrawn after flowing concrete in the pile holes toleave cast-inplace caseless piles.

10 Claims, 7 Drawing Figures U.S. Patent Dec. 16, 1975 Sheet 2 of23,925,998

METHOD FOR FORMING CAST-llN-PLACE CASELESS CONCRETE FILES BACKGROUND ANDOBJECTS OF THE INVENTION The present invention relates in general tomethods for forming castin-place caseless concrete piles, and moreparticularly to methods for forming such cast-inplace caseless concretepiles by driving into the ground a driving foot or boot memberreleasibly assembled on a hollow tubular driving mandrel to form thepile-forming hole or bore hole, then filling the bore hole with concreteand forming other pile bore-holes nearby without distortion or damage tothe previously formed hole configuration or freshly poured concrete forclosely adjacent piles.

For a long time, cast-in-place concrete piles of the type wherein asteel shell or outer casing was driven in the ground to be permanentlyleft in the ground and concrete thereafter poured therein to form thepile, found widespread utility in the construction field. However,because of the cost and shortage of steel, delays in securing deliveryof steel casings, and related factors, efforts have been made for manyyears to devise. effective methods of providing cast-in-place concretepiles which do not require a steel shell. Cast-in-place concrete pileswhich do not have such steel shells are generally termed caseless piles.The advantages to be gained by such cast-in-place caseless concretepiles lie principally in the reduction in material costs achieved byelimination of the steel shell, as a steel shell would be consumed indriving each pile and would not be recoverable, and avoidance ofscheduling problems arising from slow steel deliveries.

Some problems which have been encountered with the so-called shell-lessor caseless concrete piles are discussed in earlier US. Pat. No.3,834,304, to Upson wherein it is stated that some efforts to producecaseless type piles involved first driving a temporary metal casing orshell into the ground, then pouring concrete into the interior of thetemporary shell and pulling the shell upwardly before the concrete isset to leave a concrete pile without a protective shell. As recognizedin the prior Upson patent, this method necessarily involved certaindisadvantages, since a very heavy upward pull must be exerted to removethe casing with the result often being an upward dislodgement of thepile point which, no matter how slight, is very damaging to the ultimateconstruction since such a pile, when subsequently loaded, will sink downand cause a similar settling of the building or other load it supports.That patent described efforts made by others to eliminate that defect byreacting the upward force used to pull the casing downward on theconcrete which had been poured into that casing, but acknowledged thatsuch a method had not proven satisfactory inactual usage since thedownward reactive force when applied to the unset concrete would causethat concrete to arch and distribute a substantial component of thedownward force to the very casing one was trying to remove.

The Upson patent also discussed certain inherent disadvantages in usingthe pile-forming method employing a temporary outer casing which waswithdrawn before the poured concrete had set, especially when severalpiles are to be driven in closely adjacent positions, stating thatalthough the pile in such a cluster may initially be formed in goodcondition, as soon as the driving of a closely adjacent pile commences,the lateral walls of the first shell-less pile are subjected to severeground stresses resulting in partial or complete distortion thereof.Experience has shown that when a plurality of piles are driven inclosely adjacent positions, permanent protective shells of some sortshould be provided in order to insure perfect piles in each instance.

Further research to develop a practical method of creating acast-in-place caseless concrete pile discloses that there are threeprincipal problems involved in forming cast-in-place caseless concretepiles. When pile holes or bore holes are driven in areas relativelyclose to each other for forming a plurality of piles, a problem ariseswith the concrete itself which is poured into pile holes or cavitieswhich have been formed, since the vibrations of the driving ofsuccessive piles adversely affect the integrity of previously poured orpreceding piles. Secondly, in addition to this vibration problem,problems of necking in, pinching in or constriction of the pile orheaving of the pile itself arise. It has been observed that there canoccur necking in or pinching in or constriction of the pile diameter intwo vertically spaced regions along the piles length as it is installed,one zone of necking in or pinching in Occurring near the lower tip orlower end of the pile and the other in the zone or region of the pilenear the surface of the ground. This necking in or constriction of thepile in the lower or bottom tip region of the pile and in the upper ortop surface region of the pile is observed when pile holes have beendriven, filled with concrete, and the concrete is allowed to set'andthen the pile is withdrawn and inspected.

It is now recognized that the necking in or constriction of the pile inthe top or surface region does not occur at the same time as the neckingin of the pile near the foot or bottom. In fact, the upper neckingformation and the lower necking formation are not produced at the sametime, but are in fact the result of two different physical phenomena.The necking formation near the bottom or foot of the pile is thought tobe the result of forces produced during forming of that particular pilehole, while driving a mandrel with a releasable boot or driving footinto the soil to the proper depth and either filling concurrently withthe mandrel driving operation or after the driving boot or foot has beendriven to the proper depth. In other words, as the driving boot or footof the pile hole forming mechanism descends into the soil, the drivingforces emanating from the boot or foot are thought to cause the soildirectly beneath the foot member to rotate out from under the footmember and continue to rotate until they act inwardly on the pile holesurface at some distance above the bottom of the hole. It is therotation of the soil out from beneath the driving foot of the pile holeforming mechanism and the attempt of the soil to continue along acircular shear plane and encroach on the interface or pile hole surfaceabove the bottom hole forming foot and mandrel assembly, due to earthtransmission of forces or shock waves to a previously poured fresh pileor piles within a certain range of proximity to the location of thesuccessive pile or pile hole. As the successive or subsequent pile orpile hole forming assembly descends into the soil, the lower endgenerates forces in the soil which move laterally through the soil andattempt to squeeze adjacent freshly poured piles within a certainproximity range. This phenomena is different from the earlier mentionedsqueezing or necking in at the lower end portion of the pile or pilehole, because this squeezing in the upper region of one pile is caused,not by the driving of that pile or pile hole but by the driving of asuccesive pile or pile hole.

Consequently, three disturbing problems which have been identified inconnection with forming caseless cast-in-place concrete piles are (l)the tendency for vibration from driving successive piles or pile formingholes to disturb the concrete of earlier piles, resulting in possiblecreation of cold joints; (2) the tendency of the soil beneath the footof a pile hole forming mechanism, as it is driven, to rotate upwardlyand in to the pile hole cylinder being fonned; and (3) the tendency offorces generated by the driving foot or boot during formation of asuccessive or subsequent pile forming hole in a certain proximity tofreshly poured previous piles to squeeze the freshly poured previouspiles.

An object of the present invention is the provision of a method andapparatus for formation of cast-in-place caseless concrete pilesinvolving driving of pile forming holes or bores in the soil, wherebynecking in, pinching in or squeezing of the initially substantiallycylindrical pile forming hole of a freshly poured pile or piles duringdriving of the pile hole forming mechanism for successive or subsequentpiles within a certain proximity is avoided.

Another object of the present invention is the provision of a method andapparatus for installation of castin-place caseless concrete piles, asdescribed in the immediately preceding paragraph, whereby necking in orpinching in of an initially substantially cylindrical pile forming holeor bore in the soil in the lower region of the pile hole resulting fromtendency of the soil beneath the driving tip of a pile during driving torotate upwardly and into the pile hole cylinder is avoided.

Other objects, advantages and capabilities of the present invention willbecome apparent from the following detailed description, taken inconjunction with the accompanying drawings illustrating a preferredembodiment of the invention.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a fragmentary perspectiveview, partially in section, showing a cluster of cast-in-place freshlypoured concrete piles and a pile-forming bore hole in process offormation for another pile in the cluster, illustrating an applicationof the present invention;

FIG. 2 is a fragmentary side elevation view of a pile cluster andfooting illustrating an application of the caseless piles formed by themethod of the present invention; and

FIGS. 3 A-3 E are side elevational views, partially in section,illustrating in a sequential manner the steps for installing pilesaccording to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring to thedrawings, wherein like reference characters designate correspondingparts throughout the several figures, the present invention is concernedwith a method of forming caseless cast-in-place concrete piles in such away that where a plurality of such piles are required in relativelyclose proximity to each other, in what may be called a pile cluster,located sufficiently near to each other to produce problems of neckingin or pinching described earlier herein, operation of the pilehole-forming mechanism for pile holes subsequent to the first pile holewill not damage or produce necking or construction of the previouslypoured piles in the cluster. One example of an application of the methodof the present invention is where a plurality of caseless cast-in-placeconcrete' piles, for example five piles, are required to be formed in acluster to support one or more building footings, with each pile to belocated about four feet on centers. Such an application is illustratedin FIGS. 1 and 2, where the fifth pile hole, indicated by the referencecharacter l0e, is in process of being formed by a pile hole-formingmandrel and driving foot assembly, indicated generally at 12, whileother pile holes 10a, 10b, 10c, and 10d in the same pile cluster havealready been formed and filled with freshly poured concrete. The fivecast-inplace caseless concrete piles to be installed in this cluster,two of which, indicated as and 14b in FIG. 2, are designed to support alarge rectangular poured concrete footing 16, for example, for a portionof a large building. It will be appreciated that the problems to beovercome by the method of the present invention are also applicable to awide variety of other possible situations wherever two cast-in-placepiles are required to be formed at sufficiently small distances fromeach other that shocks or forces created during driving of the hole forthe second or subsequent pile to be formed would be transmitted withsufficient intensity to the wall of the earlier formed pile hole toproduce necking or constriction in the upper region of the earlier pilehole.

A preferred example of the mechanism and method employed to form each ofthe pile-forming holes in putting into practice the present inventionemploys a driving foot or boot member 18 having a bottom plate orleading end plate 20 and an upwardly extending hollow cylindrical sleeveor pipe portion 22 forming an upwardly opening cylindrical cavity orsocket to receive in telescoping relation therein the lower end portionof the driving mandrel 24. The diameter of the bottom plate or leadingend plate 20 of the driving foot 18 is of slightly larger diameter thanthe outer diameter of the upper sleeve portion 22, as illustrated inFIGS. l-3, in the preferred embodiment, although a driving foot whereinthe lower end portion is of the same diameter as the outer diameter ofthe upper sleeve or hollow tubular portion 22 may be employed. Also, itwill be appreciated that the bottom surface of the driving foot 18 neednot be a flat planiforrn surface, but can be a downwardly convergingpointed cone or other desired configurations, and while the driving footcontemplated in the preferred embodiment would normally be formed of aflat circular metal bottom plate to which an annular or tubular circularpipe section is welded or similarly secured, the driving foot may beformed in other ways and of other materials as will be apparent to thoseskilled in the art.

The inner diameter of the upwardly opening hollow cavity or socket 22ain the sleeve portion 22 of the driving foot is sized to closelyapproximate the outer diameter of the lower end portion of the drivingmandrel 24 to releaseably telescopically receive the lower end portionof the driving mandrel therein. In one preferred embodiment, the drivingmandrel, or elongated pusher member as it is sometimes termed, is in theform of an axially elongated hollow cylindrical pipe section having anaxial hollow bore or center opening 24a surrounded by a cylindricalouter wall 24b, which may be about 1 inch thick for a driving mandrel of8 or 10 inches outer diameter, and which is provided with a plurality ofopenings or apertures 240, which may be, for example, circular or oval,slot-like openings, at spaced locations in the cylindrical pipe wall24b. It will be appreciated that the cross-sectional configuration ofthe mandrel need not be of annular circular cross-section, but may be arectangular tubular configuration with right angular or rounded corners,or may be of other desired cross-sectional configurations providingappropriate rigidity. The cross-sectional configuration of the drivemandrel is chosen, however, in this preferred embodiment, so that thetotal cross-sectional area of the material forming the mandrel wall 24bor the body of the mandrel is much less than the cross-sectional area ofthe pile hole or cylindrical bore being formed in the earth duringdriving of the driving foot so that pile forming material, such asconcrete or the like, can readily flow by gravity from the zone at thetop of the pile hole being formed into the area outwardly I surroundingthe wall or walls of the driving mandrel and into the hollow interiorthereof and can freely flow between these two zones through the openings240 in the driving mandrel wall. The lower end of the driving mandrel 24is suitably shaped to releaseably interfit into the socket or cavity 220of the driving foot 18, and the upper end of the driving mandrel issuitably shaped to provide for releaseable connection of the drivingmandrel to the driving head components of conventional pile drivingrigs, indicated diagrammatically at 26, to permit application of impactdriving forces from the pile driving rig to the upper end of the drivingmandrel during forming of the pile hole, and to permit withdrawal of thedriving mandrel when desired from the completed pile hole. Examples ofsuitable hollow cylindrical drive mandrels with apertures in the wallthereof for producing such pile forming holes in the earth areillustrated and described in United States Patents of Jerry A. Steding,US Pat. Nos. 3,851,484 and 3,851,485 granted Dec. 3, 1974. It will beappreciated however, that many other types of driving foot members andreleasable driving mandrels may be employed to drive the driving foot orboot into the earth to the proper depth to form the vertical column boreor pile hole in the earth to be filled with the concrete or similar pileforming material, either concurrently with the driving of the pile holeor shortly after the pile hole has been formed.

When the concrete or similar fill material to form the pile is to begravity fed into the pile forming opening in the earth during thedriving of the driving boot, it is desired in the practice of onepreferred embodiment of the invention to locate a fill hopper, such asthat illustrated diagrammatically'at 28 in FIGS. 1 and 3 at grade levelat the location where the pile is to be formed, the

fill hopper having inclined side walls 28a which form downwardly andinwardly convergent sloping walls extending to a bottom opening 28b inthe hopper of substantially the diameter of the driving foot, controlledif desired by some kind of movable gate 280, so that the fill materialsuch as fiowable concrete is gravity fed downwardly through the bottomopening into the pile hole being formed in the earth during driving ofthe driving foot. The fill material thus flows into the outer annularregion 30 of the pile hole being formed, between the outer cylindricalsurface of the mandrel, such as the cylindrical driving mandrel 24, andthe substantially cylindrical walls of the column bore or pile openingbeing formed by the driving foot, and also flows inwardly through theapertures or openings 24c in the wall of the mandrel into the inner boreof the mandrel to substantially fully occupy the cross-section of themandrel bore as well as the outer annular zone immediately outwardly ofthe outer mandrel surface. The examples of a suitable fill hopperconstruction usable in practice of this embodiment of the invention arealso disclosed in the earlier Steding patent applications identifiedabove.

The ordinary procedure for forming a caseless castin-place concrete pileis to form one pile hole for a single pile in the earth by driving thedriving boot or foot with some type of mandrel structure until the footis driven to the proper depth, thereby forming a vertical cylindricalcolumn, cavity or hole in the earth, and either withdrawing the mandreland filling that pile hole with concrete, or pumping fluent concreteinto the hole thus formed while the mandrel is being withdrawn, usuallyby incorporating pressurized concrete conduits in the mandrel structureitself, or gravity feeding fluent concrete into the pile hole while thehole is being formed in the manner described previously and withdrawingthe mandrel immediately upon completion of the driving of the drivingfoot to the proper depth. In any event, in each of these procedures, themandrel is withdrawn from that pile hole before proceeding with drivingof other pile holes in the same vicinity with the same pile driving orpile forming equipment.

In order to avoid the problems of pinching or necking in of the piledetailed earlyin this application, the method of the present inventioninvolves the use of a plurality of identical mandrels and driving insuccession the boots or foot members for a plurality of piles in acluster or group located within a predetermined proximity to each other,and includesgthe important step of leaving the mandrel employed indriving each of the pile holes in the cluster or group in place fullyinserted in its respective pile hole until all of the pile holes in thecluster or in the group in predetermined proximity to each other havebeen driven, so that the mandrels left in place in each previouslyformed pile hole protect their associated pile holes from the necking inor pinching forces previously described. Differently stated, the methodof the present invention involves the forming of a plurality of caselesscast-in-place concrete piles in a cluster or group within apredetermined proximity to each other corresponding to the zone overwhich such pile hole distorting forces might occur, wherein a first oneof the pile holes is formed by driving the driving foot or boot into theearth to the proper depth with a hollow cylindrical mandrel which isreleaseably assembled with the driving foot, leaving the mandrel forsuch first pile hole fully inserted into the hole and proceeding insuccession to form the remaining pile holes in the cluster or group .ina similar manner each with its own driving foot and mandrel, in eachcase leaving the mandrel fully inserted in its associated pile holeuntil all of the pile holes of the cluster or group are formed. The pileholes can either be filled with concrete currently with driving of thedriving foot and mandrel into the pile hole, or can be filled withconcrete after each pile hole is formed or after all of the pile holesin the cluster or group are formed. It will be appreciated that wherethe maximum time required to form the pile holes for all of the piles ofthe cluster or group is such that the concrete introduced into theearliest driven pile holes concurrently with or immediately afterdriving such earliest holes would have set to such an extent as toprevent withdrawal of their associated mandrels without damage to theassociated pile after all of the pile holes are formed, then filling ofthe pile holes with concrete should be deferred until the last pile holeor the last few pile holes have been formed. In any event, when the pileholes forming the cluster or group have been filled with concrete, themandrels are then promptly removed from each of the filled pile holes.The effect of leaving the mandrels in the pile holes until all of thepile holes in the cluster or group have been driven is that the mandrelthen serves as a protective casing or body within the driven pile holeto protect the pile hole and the concrete within that pile hole if ithas already been poured, from the destructive forces giving rise to thepinching or necking in of the piles in the upper region and in the lowerregion of the pile. Once the mandrel is installed in the pile hole, itsstructural rigidity serves to protect the pile hole and theconfiguration thereof, and the concrete within that pile hole after ithas been introduced into the pile hole, from the forces of subsequentdriving of closely adjacent pile holes as well as from forces causedduring driving of that pile hole itself.

As one example, assuming it is desired to install five 12 inch diametercaseless cast-in-place concrete piles in a five pile cluster, forexample, to support a footing such as the footing 16 illustrated in FIG.2, with each pile located four feet on centers, and each pile to beapproximately 35 feet long, five pile forming foot members or boots 18and five mandrels 24 would be required. The assembled foot member andmandrel 12 for each pile hole would be driven in succession to form therespective pile holes a, 10b, 10c, 10d, and Ne, and the pile holes mayeither be left unfilled until the last pile hole, for example pile hole10a, has been driven or they may be filled with concrete concurrentlywith the driving of each pile hole or immediately following the drivingof each pile hole. In any event, the mandrels 24 are left fully insertedin each of the pile holes 10a, to We until the fifth or last pile holehas been driven, after which the mandrels are then removed, preferablyafter previously filling of the pile holes with concrete. This procedureinsures that the pile hole or column cavity formed for each pile by thedriving of the foot or boot is protected against concurrent andsubsequent pinching or squeezing forces until the last boot and mandrelassembly has been installed and there are no subsequent driving forcesacting on the five pile group. After the mandrels for the five pilegroup have been extracted, in effect uncasing the piles, what remainswill be the five caseless cast-in-place concrete piles each installed ina manner protecting the pile hole from the pinching or pile constrictingforces which 8 would adversely affect the load bearing capacity andintegrity of the pile.

What is claimed is:

1. The method of forming plural cast-in-place caseless concrete piles insuch proximity area to each other that earth transmission of forcesduring driving of some of the pile holes may damage the shape of otherpile holes already formed in the proximity area, comprising the steps offorming a first pile hole in the earth by driving to the desired depth afirst hole forming assembly including a first driving foot having aportion of cross-sectional shape and size corresponding to the desiredcross-sectional shape and size of the pile hole and a first elongateddriving mandrel of hollow tubular configuration releasably coupled tothe driving foot and having a tube wall surrounding the hollow interiorof the mandrel of slightly smaller size than the transverse size of thepile hole locating the outer surface of the tube wall sufficiently nearthe pile hole wall surface to protect the latter against inwarddistortion, temporarily leaving said first driving mandrel and drivingfoot in said first pile hole driven to said predetermined depth andforming a second pile hole located in said proximity area by driving ahole forming assembly of a second driving foot and a second drivingmandrel of substantially the same construction as said first drivingfoot and mandrel to the desired depth in the earth while the firstdriving mandrel remains fully inserted in its respective pile holeprotecting the shape of the first pile hole throughout its heightagainst distorting forces, filling the formed pile holes with concrete,and withdrawing the driving mandrels from the pile holes before anyconcrete therein has set sufficiently such that mandrel withdrawal woulddamage the concrete, thereby leaving a cast-in-place caseless concretepile in each pile hole.

2. The method of forming plural cast-in-place caseless concrete piles asdefined in claim 1, wherein said first pile hole is filled with concretewhile said first driving foot and mandrel are being driven to saidpredetermined depth.

3. The method of forming plural cast-in-place caseless concrete piles asdefined in claim 1, wherein said first pile hole is filled with concretewhile said first driving foot and mandrel are being driven to saidpredetermined depth by concurrently gravity feeding flowable concretedownwardly into the hollow interior of and the space outwardlysurrounding said tubular mandrel progressively during driving of themandrel.

4. The method of forming plural cast-in-place caseless concrete piles asdefined in claim 1, including the steps of locating a mobile hopperhaving a bottom opening sized to correspond substantially to the desiredpile hole diameter and downwardly convergent sloping sides convergingsubstantially to said opening over each respective pile site where apile hole is to be formed, vertically alining the bottom opening of saidhopper with the center axis of the pile hole to be formed, feedingflowable concrete adequate to form the pile into the hopper, and drivingthe mandrel for each pile hole along a vertical axis downwardly throughthe concrete in the hopper and through the bottom opening into the earthto flow concrete from the hopper into the hollow interior and the outersurrounding regions of the mandrel concurrently with the drivingthereof.

5. The method of forming plural cast-in-place case less concrete pilesas defined in claim 4, wherein said 9 driving mandrel has plural radialopenings in the tube wall thereof for passage of fiowable concretebetween said outer surrounding region and said hollow interior of themandrel during driving of the mandrel.

6. The method of forming plural cast-in-place caseless concrete piles asdefined in claim 5, wherein said tube wall of said driving mandrel is ofsufficient thinness as to occupy only a minor fraction of the totalcross-sectional area of the pile hole being formed affording maximumvoid space for gravity flow of the flowable concrete downwardly alongand within the mandrel during driving thereof.

7. The method of forming plural cast-in-place caseless concrete piles asdefined in claim 1, wherein said step of filling the pile holes withconcrete is performed only after all of the pile holes in said proximityarea have been formed, and said mandrels are withdrawn promptly uponfilling of the respective pile hole which it occupied.

8. The method of forming plural cast-in-place caseless concrete piles ina cluster in such proximity area to each other that earth transmissionof forces during driving of some of the pile holes of the cluster maydamage the shape of other pile holes of the cluster already formed inthe proximity area, comprising the steps of forming a first pile hole inthe earth by driving to the desired depth a first hole forming assemblyincluding a first driving foot having a portion of crosssectional shapeand size corresponding to the desired cross-sectional shape and size ofthe pile hole and a first elongated driving mandrel of hollow tubularconfiguration releasably coupled to the driving foot and having a tubewall surrounding the hollow interior of the mandrel of slightly smallersize than the transverse size of the pile hole locating the outersurface of the tube wall sufficiently near the pile hole wall surface toprotect the latter against inward distortion, temporarily leaving saidfirst driving mandrel and driving foot in said first pile hole driven tosaid predetermined depth and forming each successive pile hole in thecluster occupying said proximity area by driving a respective holeforming assembly of a driving foot and a driving mandrel for eachsuccessive pile hole having substantially the same construction as saidfirst driving foot and mandrel to the desired depth in the earth whilethe driving mandrel for the first and each previously formed hole of thecluster remains fully inserted in its respective pile hole protectingthe shape of previously formed pile holes throughout their heightagainst distorting forces, filling the formed pile holes with concrete,and withdrawing the driving mandrels from all of the pile holes in thecluster before any concrete therein has set sufficiently such thatmandrel withdrawal would damage the concrete, thereby leaving acast-in-place caseless concrete pile in each pile hole.

9. The method of forming plural cast-in-place caseless concrete piles asdefined in claim 8, wherein said first pile hole and each successivepile hole in the cluster is filled with concrete while its respectivedriving foot and mandrel are being driven to said predetermined depth byconcurrently gravity feeding flowable concrete downwardly into thehollow interior of and the space outwardly surrounding said tubularmandrel progressively during driving of the mandrel.

10. The method of forming plural cast-in-place caseless concrete pilesas defined in claim 8, including the steps of locating a mobile hopperhaving a bottom opening sized to correspond substantially to the desiredpile hold diameter and downwardly convergent sloping sides convergingsubstantially to said opening over each respective pile site where apile hole is to be formed, vertically alining the bottom opening of saidhopper with the center axis of the pile hole to be formed, feedingflowable concrete adequate to form the pile into the hopper, and drivingthe mandrel for each pile hole along a vertical axis downwardly throughthe concrete in the hopper and through the bottom opening into the earthto flow concrete from the hopper into the hollow interior and the outersurrounding regions of the mandrel concurrently with the drivingthereof, each driving mandrel having plural radial openings in the tubewall thereof for passage of flowable concrete between the outersurrounding region and said hollow interior of the mandrel duringdriving thereof.

1. The method of forming plural cast-in-place caseless concrete piles insuch proximity area to each other that earth transmission of forcesduring driving of some of the pile holes may damage the shape of otherpile holes already formed in the proximity area, comprising the steps offorming a first pile hole in the earth by driving to the desired depth afirst hole forming assembly including a first driving foot having aportion of cross-sectional shape and size corresponding to the desiredcross-sectional shape and size of the pile hole and a first elongateddriving mandrel of hollow tubular configuration releasably coupled tothe driving foot and having a tube wall surrounding the hollow interiorof the mandrel of slightly smaller size than the transverse size of thepile hole locating the outer surface of the tube wall sufficiently nearthe pile hole wall surface to protect the latter against inwarddistortion, temporarily leaving said first driving mandrel and drivingfoot in said first pile hole driven to said predetermined depth andforming a second pile hole located in said proximity area by driving ahole forming assembly of a second driving foot and a second drivingmandrel of substantially the same construction as said first drivingfoot and mandrel to the desired depth in the earth while the firstdriving mandrel remains fully inserted in its respective pile holeprotecting the shape of the first pile hole throughout its heightagainst distorting forces, filling the formed pile holes with concrete,and withdrawing the driving mandrels from the pile holes before anyconcrete therein has set sufficiently such that mandrel withdrawal woulddamage the concrete, thereby leaving a cast-inplace caseless concretepile in each pile hole.
 2. The method of forming plural cast-in-Placecaseless concrete piles as defined in claim 1, wherein said first pilehole is filled with concrete while said first driving foot and mandrelare being driven to said predetermined depth.
 3. The method of formingplural cast-in-place caseless concrete piles as defined in claim 1,wherein said first pile hole is filled with concrete while said firstdriving foot and mandrel are being driven to said predetermined depth byconcurrently gravity feeding flowable concrete downwardly into thehollow interior of and the space outwardly surrounding said tubularmandrel progressively during driving of the mandrel.
 4. The method offorming plural cast-in-place caseless concrete piles as defined in claim1, including the steps of locating a mobile hopper having a bottomopening sized to correspond substantially to the desired pile holediameter and downwardly convergent sloping sides convergingsubstantially to said opening over each respective pile site where apile hole is to be formed, vertically alining the bottom opening of saidhopper with the center axis of the pile hole to be formed, feedingflowable concrete adequate to form the pile into the hopper, and drivingthe mandrel for each pile hole along a vertical axis downwardly throughthe concrete in the hopper and through the bottom opening into the earthto flow concrete from the hopper into the hollow interior and the outersurrounding regions of the mandrel concurrently with the drivingthereof.
 5. The method of forming plural cast-in-place caseless concretepiles as defined in claim 4, wherein said driving mandrel has pluralradial openings in the tube wall thereof for passage of flowableconcrete between said outer surrounding region and said hollow interiorof the mandrel during driving of the mandrel.
 6. The method of formingplural cast-in-place caseless concrete piles as defined in claim 5,wherein said tube wall of said driving mandrel is of sufficient thinnessas to occupy only a minor fraction of the total cross-sectional area ofthe pile hole being formed affording maximum void space for gravity flowof the flowable concrete downwardly along and within the mandrel duringdriving thereof.
 7. The method of forming plural cast-in-place caselessconcrete piles as defined in claim 1, wherein said step of filling thepile holes with concrete is performed only after all of the pile holesin said proximity area have been formed, and said mandrels are withdrawnpromptly upon filling of the respective pile hole which it occupied. 8.The method of forming plural cast-in-place caseless concrete piles in acluster in such proximity area to each other that earth transmission offorces during driving of some of the pile holes of the cluster maydamage the shape of other pile holes of the cluster already formed inthe proximity area, comprising the steps of forming a first pile hole inthe earth by driving to the desired depth a first hole forming assemblyincluding a first driving foot having a portion of cross-sectional shapeand size corresponding to the desired cross-sectional shape and size ofthe pile hole and a first elongated driving mandrel of hollow tubularconfiguration releasably coupled to the driving foot and having a tubewall surrounding the hollow interior of the mandrel of slightly smallersize than the transverse size of the pile hole locating the outersurface of the tube wall sufficiently near the pile hole wall surface toprotect the latter against inward distortion, temporarily leaving saidfirst driving mandrel and driving foot in said first pile hole driven tosaid predetermined depth and forming each successive pile hole in thecluster occupying said proximity area by driving a respective holeforming assembly of a driving foot and a driving mandrel for eachsuccessive pile hole having substantially the same construction as saidfirst driving foot and mandrel to the desired depth in the earth whilethe driving mandrel for the first and each previously formed hole of thecluster remains fulLy inserted in its respective pile hole protectingthe shape of previously formed pile holes throughout their heightagainst distorting forces, filling the formed pile holes with concrete,and withdrawing the driving mandrels from all of the pile holes in thecluster before any concrete therein has set sufficiently such thatmandrel withdrawal would damage the concrete, thereby leaving acast-in-place caseless concrete pile in each pile hole.
 9. The method offorming plural cast-in-place caseless concrete piles as defined in claim8, wherein said first pile hole and each successive pile hole in thecluster is filled with concrete while its respective driving foot andmandrel are being driven to said predetermined depth by concurrentlygravity feeding flowable concrete downwardly into the hollow interior ofand the space outwardly surrounding said tubular mandrel progressivelyduring driving of the mandrel.
 10. The method of forming pluralcast-in-place caseless concrete piles as defined in claim 8, includingthe steps of locating a mobile hopper having a bottom opening sized tocorrespond substantially to the desired pile hold diameter anddownwardly convergent sloping sides converging substantially to saidopening over each respective pile site where a pile hole is to beformed, vertically alining the bottom opening of said hopper with thecenter axis of the pile hole to be formed, feeding flowable concreteadequate to form the pile into the hopper, and driving the mandrel foreach pile hole along a vertical axis downwardly through the concrete inthe hopper and through the bottom opening into the earth to flowconcrete from the hopper into the hollow interior and the outersurrounding regions of the mandrel concurrently with the drivingthereof, each driving mandrel having plural radial openings in the tubewall thereof for passage of flowable concrete between the outersurrounding region and said hollow interior of the mandrel duringdriving thereof.